1. Field of the Invention
The present invention relates to a method of controlling an amount of light in an optical scanning apparatus used in an image forming apparatus.
2. Description of the Related Art
In an electrophotographic-type image forming apparatus, an important factor in terms of maintaining image quality is to perform precise control of the amount of light in an optical scanning apparatus that outputs a laser beam for forming an electrostatic latent image. Generally, in control of the amount of light, what is controlled is the relationship between a driving current applied to the light source and the amount of light in the light beam emitted from the light source.
In an optical scanning apparatus, generally the FFP (Far-Field Pattern) and optical axis fluctuate in accordance with differences in image height (scanning position in a main-scanning direction of photosensitive member) and amount of light. On the other hand, in accordance with Japanese Patent Laid-Open No. 2005-262485, one scanning line is divided into a plurality of correction intervals and correction data in each correction interval is determined. Further, Japanese Patent Laid-Open No. 2005-262485 proposes that correction data applied to any image height be calculated by interpolation processing and that the amount of light be calculated using the calculated correction data.
In accordance with the invention described in the specification of Japanese Patent Laid-Open No. 2005-262485, the amount of light with which a photosensitive drum is irradiated can probably be made uniform. However, in order to achieve a greater improvement in image quality, a uniform surface potential (dark potential or highlight potential) of the latent image formed on the photosensitive drum is required. If the surface potential cannot be made uniform, an unevenness in density will occur when the latent image is developed as by toner. This is undesirable.
In general, obtaining a constant sensitivity characteristic in all areas on the surface of a photosensitive drum is exceedingly difficult in terms of the manufacturing process. The reason is that since a photoconductive layer is formed on the photosensitive drum while the photosensitive drum is being rotated, the film thickness of the photoconductive layer cannot be rendered constant owing to uneven vacuum deposition. Preferably, therefore, the amount of light in each area of the drum surface should be corrected taking into consideration the fact that the sensitivity characteristic of the photosensitive drum is not uniform.